Method of producing cast refractory and similar articles



Jul 21, 1936.

R. C. BENNER ET AL METHOD OF PRODUCING CAST REFRACTORY AND SIMILAR ARTICLES Filed Feb. 25, 1933 2 Sheets-Sheet l RAYMOND C. BENNER By GEORGE I EASTER A TTORNEYS.

July 2 1, 1 936. R. c. BENNELR ET AL 2,048,319

METHOD OF PRODUCING CAST REFRACTORY AND SIMILAR ARTICLES Filed Feb. 25 1933 2 Sheets-Sheet 2- V n d no N w i M I- -'9i" Fla 4- FIG.

- INVENTOR. Y RAYMOND c. BENNER areas: a: EASTER ATTORNEYS.

v ever. permit rarss METHOD OF PRODUCI TORY AND no. clis'r REFRAC- narrows Raymo nd C. Bennetand George J. Easter, Niagara Falls, N. Y., assignora to The Carborundum Company, Niagara Falls, N. Y.,

of Pennsylvania a corporation Application February 25, 1933, Serial No. 658,534 s Claims. (Cl. 49-79) 1:: z. invention relates to an improvement in the method of producing cast refractory and similaiarticles from molten oxides and thelike, and is particularly concerned with the method of 5 molding and cooling such cast articles.

In the nzanufacture of glass tank blocks and other refractories as hitherto practiced, the component materials have been fused in an arc'type furnace and from thence tapped into a sand mold or the like and the casting then covered with infusorial earth or the like to cool. It has, however, been found that this practice produces two types of defects in the castings so made. First, it is very cult to overcome the tendency to form hollow "pipes" extending down toward the center of the casting from its upper surface. Second, regardless of this piping tendency, it has been found that blocks of the size ordinarily used for glass tanks, and even smaller pieces, frequently crack intopieces perhaps four incheson a side as a result of cooling and shrinkage of the outside of the piece at a more rapid rate than the 1 inside.

In order to overcome this difficulty, we have attempted to preheat the molds for the blocks in a some or similar furnace. We have, however, experienced such difficulty in thus handling the molds that we have concluded that in order to secure the advantages of such a. method it would i be necessary to build a tunnel kiln in which the molds would be placed on carspreheated, filled through a suitable opening in the kiln, and then cooled slowly while still in the' kiln.

The interior of themolds may be preheated w. electrically by inserting an electrical resistance heater made of siliconized or recrystallized silicon carbide rods (which may be made by methods described in detail in the U. S. Patent ti-1,906,853

to Hediger and in the U. .8. Patent #1906363 to do Heyroth) within the mold while supporting the resistor on the mold cover, and bringing thetem perature. up to the desired point by passing electric current through the resistor. This method of preheating the interior of the mold directly permits molds to be made of cheap material, as for instance, a backing of sand which may if desired be faced with some more refractory material. It ofiers a considerably easier way of preheating the interior of the mold than is possible by inserting mold in a furnace. It does not; howas close control of the rate of cooling is desirable. and as we have found possible by means now to be described.

An economical and easily workable solution of the whole both the mold preheating and annealing problems has been found wherein molds made of refractory materials, such as graphite or silicon carbide, have been heated electrically either by induction or resistance heating, the former being somewhat preferable on account of its greater 5 flexibility of operation and mobility of apparatus. In this way it is possible to preheat the mold to any desired degree, hold it at a desired temperature to fill it, and retard cooling to a degree permitting the difficulties encountered under the old method to be readily overcome.

In the accompanying drawings, Figure l is a diagrammatic view of an inductively heated mold set on a car, into which molten material is being poured from a tilting arc furnace;

Figure 2 is a vertical section through the center of a mold such as we use, placed in a coil for inductive heating;

Figure 3 is a vertical section through the center of asimilar mold arranged for resistance heating; and

Figure 4 is a vertical section through the center of a furnace in which both the mold and the material to be cast are heated inductively.

In the production of fused alumina, mullite, spinel, or the like to be cast into blocks for use as refractories or otherwise, we may use an arc furnace of the ordinary type used in the production of fused alumina abrasives in which a pot of bauxite or like material is fused in a water cooled iron shell and thereafter we tap or pour this fused material into our molds.

Figure 1 shows diagrammatically such an arc furnace I mounted in trunnions 2 and provided with electrodes 3 which in the drawings are indicated as raised out of the pot to permit pouring. 'The pot is tipped by means of a cable 4 in such a way that the molten material runs out of spout 5 and down an inclined trough 6 into the mold contained in an inductively wound coil 1, one or 40 more of which molds are mounted on a truck 8. When one mold is filled, the cable 8 is slacked off while another moldis placed in position and the second one filled in a similar manner, spout 5 being cleared of congealed material as often as need be by the use of a crowbar. Other tapping. arrangements can obviously be used in place .of the tilting of the pot furnace.

Figure 2 shows in detail a section through the mold shown in Figure l at a time when the mold is empty and is arranged forpreheating. The inductively wound coil 5 has water circulating through it, preferably impelled by suction rather than by positive pressure in order that chances of an explosion may be minimized. The water connections and electrical connections to this not been illustrated, in detail. Within the coil and may be lined or coated with a silicon carbide 'blocks I3 made of silicon carbide there is a'cylinder 9 of fused quartz, zircon or other suitable material mounted in grooves ill in a commition asbestos board It, Inside this cylinder the mold i2 is centrally supported upon or other suit: able refractory. The mold is made 01' graphite mixture or other suitable compound to aid in resisting oxidation. The space surrounding the mold is filled with a thermal insulator such as lamp black l6. Above the crucible l2 and cylinder 9, a refractory collar i5 is placed in order to protect the various pa of the coil in the in:- nace from splashing molten oxides at the time that the mold is being filled. During the preheating stage, a cover it of insulating brick is I placed over the mold in order to conserve heat.

' mold hasbeen suficiently to the desired depth. The

- placed loosely across the When the molten material is ready and the preheated, the latter is placed in position adjacent the furnace containing the molten materiel, cover it removed and the molten material poured into the mold guardring i5 is then graphite cover, not shown, is top of the crucible it with insulating material such as lamp black this cover.

removed and a the unit as a whole in audio conserve heat.

The coil with the crucible within it is then moved along so that another mold in another order to reduce oxidation l coil may be filled, etc.

- filled is maintained 4Q erably, be tapered outward slightly toward the The temperature of the mold which has been by continuing to induce current into the walls of the mold. It is, however, sraduallyallowed to cool oil at such a-rate that practically. the entire cast block solidifies simultaneously. In this way the upper surface of the poured material is kept sufllciently fluid to prevent serious cracking and alter the whole block has solidified it, is cooled very gradually so, that the temperature gradient from inside to outside does not exceed 50 C. "per'inch until the temperature is sumcie'ntly low for the block to be removed from the mold. The mold should, preftop so that the block can be removed without the necessity for disassembling the mold. Figure 3 illustrates an alternative construction which, while serviceable, is somewhat more cumbersome. to manipulate, particularly where a large number of molds are to be-handled' simultaneously. In accordance with this construction, the mold l2a is provided with a. refractory cover I! and an insulating cover 1 sembly-being mounted in a silicon carbide cylinder it, which is illustrated as surrounding the mold so closely as to aid in supporting it. Both the mold and this'cylinder should be tapered outward slightly toward the top to facilitate the removal of the cast piece; In this arrangement the heat is generated by passing current through granular graphite 20 surrounding the cylinder ll; current being introduced at opposite ends oi' the furnace through graphite electrodes 2|. The

' granular graphite is enclosed on the outside by an additional silicon carbide cylinder 22 and silicon carbide fire-sand 23 is used to thermally insulate the outside of the entire assembly. Other suitable materials might obviously be sub- A second cover,

It, the entire. as-

filled all the molten material is out of the cru-'- accents stituted for those mentioned if desired In utilizing this type of equipment, the covers are removed while the mold is being filled and are then replaced in the position shown. Otherwise, the operation is practically similar to that described in connection with Figure 2. A As an alternative method of procedure, the material which is to be made into a solid block may be placed directly in a mold as shown in Figure 2 or 3 in granular form and melted directly therein, cooling being done as above described' Figure 4 shows an alternative device in which the material is melted in a crucible 26 arranged to be inductively heated by the. surrounding coil 1 which is illustrated ,as separated from the thermal insulating material within it by a mica sheet 93. Connectedto this crucible is mold lZB arranged to be similarly heated. Plug 25, which is provided with a vent 26 along itsaxis and is tapered slightly at the bottom, 'is provided to prevent premature flowing of the molten material from the crucible into the mold; Collar 27, which is split vertically into 'two or'more sections, is used to connect the crucible and the mold. a-A 25 the plug 251s raised, the molten material contained in the crucible flows down around the outside of plug 25 into the mold and the air from the mold escapes through vent 26. Small vents 28 provided near the corners of the mold w; provide for the escape' or any entrapped air in those portions of the mold, but are not large enough for molten material to flow into them. Thejoints between the various pieces of graphite are sealed before the iumace is started by means' 01 a paste-made of powdered graphite and a te'm porary binder such as dextrin. The mold is illustrated as surrounded by granular carbon MB rather than'by lamp black It as is the crucible since he carbon is a poorer insulator and it is de 40 sirable that the mold be slightly cooler than the container from which the material is fed to it: The amount 01' material originally placed in the crucible' is so adjusted that when the mold is clble and its upper surface is within the con-'- ture gradient as 4 necting passage between the mold and the cru+ clble where'it remains molten and fill'sin the center of the piece below as shrinkage occurs; thus giving added protection against the forma 50" tion of cracks and'giving a. solid cast piece; The mold may be readily separated from thecollar after casting has been completed, although it' is desirable to allow the coil and its contents to remain undisturbed and cool gradually as indicated previously. v v

For the. purposes of inductively heating the molds, we utilize a coil to which rapidly alternat-'- ing potential is applied, the numberof alterna tions being'from 1,000 to 50,000 persecond. As noted above, we find that one of the principal requirements for securing cast blocks free from cracks is maintain as small a tempera possible from the center to the outside of the cast piece. This is particularly Blivltal during the stage where the'entire block is solidifyi but even after that it is necessary to cool theblocks so slowly as to maintain very small temperature gradients.v 'At the moment of vsolidiflcationythe temperature gradient 01' 50 Q./inch should not be materially exceeded. This value may be determined in test runs .by meas 'uring the thicmiess of shell between a spot on the outside of the shell where inner edge or the flow out, the inner surface so lef mold continue to be 'lating material and also a,o4s,a1o shell which is left. whenthe material composing suddenly allowed to being at a eezing point the center of the casting is temperature correspondingto the f of the material. In order to maintain such small temperature gradients, it is necessary that the heated after the material is poured into it, although at a decreased rate. It is readily possible to manipulate a series of coils and molds with a single alternator in such a way that thetemperature gradient in each is controlled to the desired amount without interference with the others. The mechanism for such control is well known to the art and we' have not, therefore, felt that it is necessary to describe it as it constitutes no part of our invention. It is somewhat more dimcult to estimate the temperature gradient in the piece after the mid- -dle has solidified, although we believe that a gradient not over C.-/inch is the maximum which can be safely permitted at any time during the cooling of the cast-piece. r

We have further forindthat in case the tem- I perature gradient becomes so great as to cause the piece to crack, it is sometimes possible to heat the mold to a temperature which causes the piece to again coalesce. This, howeve is a very slow process and it is in general better to prevent the formation of cracks rather than to try to cure them after they have formed.

Having thus described our invention, what we claim is:

L'Apparatus for producing solid refractory castings comprising a graphite crucible arranged above a graphite mold, a duct connecting the crucible with the mold, a primary coil distributed around the crucible and mold for the passage of a high frequency alternating cinrent which causes heating of both the crucible and the mold, a hollow plug extending through the crucible and disconnecting the crucible from the mold when the plug tion, and permitting the flow of molten material from the crucible to the mold in the raised positionof said plug while eillux of air takes place from the center of the mold through the hollow plug, and small orifices in the sides of the mold which permit eillux of gas from the sides of the mold without substantial loss of liquid.

, 2. The art of, castingfrefractory'articles in close succession which comprises mounting a series of .electricallyconducting molds so that each is surrounded laterally by thermally insuby a primary induction coil, heating each mold by induction and passing it adjacent to a supply of molten refractory when the temperature h reached a predetermined value, filling the moi from the supply of molten refractory. and gradually diminishing the heating afterthe mold has. been filled with moltenrefractory and'covered with insulating material.

3. Means for casting dense refractory articles is in its lowest posia conducting crucible mounted above a primary induction coil surrounding the crucible, a primary induction coil similarly surrounding the mold, a hollow cylinder connecting the bottom of the crucible with the top der fitting into the first hollow cylinder and projecting up through the crucible to shut off the connectionbetween thecrucible and the mold when said second hollow cylinder is in its lowest position, whereby slight elevation of the second cylinder permits fiow of molten refractory into the mold and also permits outfiowof air from comprising a conducting mold,

the'unfllledportion of the mold throush said u second cylinder as the mold is being filled.

4. The steps in the method of casting refractory articles which comprise mounting an electrically conducting fractory material above an electrically conducting mold, heating the conducting bodies by subjecting them to the rapidly varying magnetic field until .the refractory material has melted and subsequently thereto, and transferring the molten material by downward fiow around a central vent through which the displaced air fiows during "the process of casting.

. 5. The steps in the method of casting refractory articles which comprise mounting an electrically conducting crucible containing refrac-' tory material abovean electrically conducting mold, heating the conducting bodies by subjecting them to similar rapidly varying magnetic fields from a common source until the refractory Y has melted and for some time thereafter, transferring the molten material by downward fiow adjacent a central vent through which the displaced air flows during the process of casting, and dissipating the heat slightly more rapidly from the mold than from the crucible. I

8. Apparatus for casting successively a series 401 of articles composed of high melting refractory, said apparatus comprising an electric furnace mounted for pouring such refractory material in molten condition, a series of molds mounted for transportation to and from the outlet of said furnace, each mold having a graphite receptacle lined on the inside with oxidation resisting material, an insulating layer of finely divided packing material beneath said receptacle and around the sides thereof, a primary induction coil mounted outside ofsaid insulating layer, a refractory collar ering the insulating layer and project'- ing into the mold to catch ejected refractory material, and an insulating cover supported on said collar and sealing the mold on its upper side, whereby each mold of the series can be preheated prior to being filled with molten refrac- 'toryand heated at a gradually decreasing rate subsequent tothe pouring df the Gmonqn J.

of the mold. and a second hollow cylincrucible containing rea l RAYMOND C. 

